In recent years, greater expectations have been placed on thermoelectric conversion elements due to a circumstance in which efforts against environment and energy problems have been actively pursued for realizing a sustainable society. This is because heat is the most general energy source that can be obtained from various media, such as the body temperature, the sunlight, engines and industry exhaust heat. For this reason, it is expected that thermoelectric conversion elements will become increasingly important in future in the purposes of attainment of the highly-efficient energy use in the low-carbon society and the electric power supply to ubiquitous terminals, sensors and the like.
Thermoelectric conversion elements known in the art include those which use the Seebeck effect to generate electric power and those which use the Peltier effect to achieve cooling and heating. A thermoelectric conversion element which uses the Seebeck effect to generate electric power is disclosed in, for example, patent literature 1 (JP 2010-205883 A). The thermoelectric conversion element disclosed in this literature is composed of a plurality of pillar members and a coupling member which couples the pillar members. The pillar members are jointed to a high temperature side electrode, and the coupling member is jointed to a low temperature side electrode. Both of the joint surfaces on which the pillar members are jointed to the high temperature side electrode and on which the coupling member is jointed to the low temperature side electrode are flat surfaces. Also, a thermoelectric conversion element that uses the Peltier effect to achieve cooling and heating is disclosed in, for example, patent literature 2 (JP 2007-93106A). This literature discloses a heat conversion apparatus which uses a corrugated fin. Also in this heat conversion apparatus, the joint surface on which a thermoelectric element and an electrode are jointed is a flat surface.
In recent years, the spin-Seebeck effect, in which a flow of electronic spins is generated when a temperature gradient is applied to magnetic material, has been discovered, and thermoelectric conversion elements have been proposed which convert a flow of angular momentums (or a spin current) generated by the spin-Seebeck effect into an electric current (electromotive force) to be extracted by using the inverse spin-Hall effect. Thermoelectric conversion elements which use the spin-Seebeck effect and the inverse spin-Hall effect to generate electric power have a probability to achieve a conversion efficiency higher than those of thermoelectric conversion elements which use the Seebeck effect, and technical researches and developments have been progressed.
A thermoelectric conversion element which uses the spin-Seebeck effect and the inverse spin-Hall effect is disclosed in, for example, patent literature 3 (JP 2009-130070A) and non-patent literatures 1 and 2. The thermoelectric conversion element disclosed in the patent literature 3 is composed of a ferromagnetic metal film formed by a sputtering method and a metal electrode. In this configuration, when a temperature gradient in the direction parallel to the in-plane direction of the ferromagnetic metal film is applied, a spin current is induced in the direction along the temperature gradient by the spin-Seebeck Effect. The induced spin current can be externally extracted as an electric current by the inverse spin-Hall effect in the metal electrode which is in contact with the ferromagnetic metal film. This allows achieving a temperature difference power generation to extract electric power from heat.
The thermoelectric conversion elements disclosed in non-patent literatures 1 and 2, on the other hand, are composed of a magnetic insulator and a metal electrode. More specifically, non-patent literature 1 reports a thermoelectric conversion based on a temperature gradient parallel to the in-plane direction of the magnetic insulator (that is, the in-plane temperature gradient), similarly to patent literature 3. Also, in non-patent literature 2, the thermoelectric conversion is actually proved by an arrangement with a temperature gradient perpendicular to the plate surface (a temperature gradient in the direction perpendicular to the surface) of the magnetic insulator with a thickness of 1 mm.
It is desirable that a thermoelectric conversion element which uses the spin-Seebeck effect and the inverse spin-Hall effect exhibits a high thermoelectric conversion efficiency. Increased electric energy can be extracted from reduced heat energy as the efficiency of the thermoelectric conversion is increased.